Ron Guth: No gold commemorative coins were produced in 1990. In 1991, the Mint commemorated the 50th anniversary of the Mt. Rushmore monument by issuing specially designed Half Dollars, Silver Dollars, and Five Dollar gold pieces. The obverse of the $5 gold featuresan eagle flying towards the monument; the reverse has the words "MOUNT RUSHMORE NATIONAL MEMORIAL" in fancy script. Both Mint State and Proof versions were sold to collectors at a premium over the gold value. All of the Mt. Ryshmore commemorative gold coins were struck at the West Point Mint.
This is a relatively common coin that can be located with ease in superb condition.